Constant PENDING_PID_TTL_MS 

const PENDING_PID_TTL_MS: i64 = _; // 300_000i64

How long a stashed pid waits for its row before being dropped. Generous relative to the observed race window (a network round-trip plus one SQLite write, normally well under a second) but still bounded — a declared-background call whose launch is never actually accepted as background client-side (e.g. the command finished fast enough that Claude’s harness returned synchronously despite run_in_background: true — see issue #2519) would otherwise leak an entry here forever, since background_task_observe would then never fire for that id.